So, in theory, if you can reach the minimum speed of the cruise control system (usually around 25 m/h or around 40 km/h), and your vehicle has a cruise control system, you should be able to engage it. But since driving in reverse is akin to driving in first gear, sustaining that speed if you can reach it would probably destroy your engine (high RPMs) or transmission, or drive line.

However, if anyone cares to try it, please /msg me with the results and I will be more than happy to update my w/u. (I will not be able to reimburse you for damages).